The present invention relates to lumped constant non-reciprocal circuit elements such as circulators and isolators for use in VHF, UHF and microwave frequency bands.
A circulator can be converted into an isolator if one port is closed with a dummy resistor, and an isolator can be converted into a circulator if one port is used as a connection port for external networks. Namely, they are of the same structure, though they are called differently depending on how they are used. Thus, lumped constant circulators and isolators have the same technological basis. Hence, it should be noted that though isolators are explained hereinafter, substantially the same explanations are applicable to circulators.
A typical prior art isolator is shown in a cross sectional view in FIG. 7. The isolator has three sets of central conductors 201 positioned between two garnet plates 202 which are insulated from the conductors by means of an insulator, and these elements are covered with a shielding plate 203. They are placed in the central bore of a ceramic substrate 204 with one end of each conductor 201 connected to a corresponding electrode (not shown) provided on the ceramic substrate 204. A permanent magnet 206 coated with an iron plate 205 is provided above the garnet plate 202. All of these elements are housed by the casing 207 and 208. Other elements such as hollow coils and capacitors may be implemented therein if required. Two ceramic substrates are often laminated.
Such prior art isolators have been made compact followig the miniaturizaiton of other microwave elements. For example, a very small isolator having an area of 20 mm.times.20 mm and a height of 10 mm has been realized, which uses garnet plates of 10 mm in outer diameter and 0.9 mm in thickness, and a permanent magnet of 13 mm in outer diameter and 4 mm in thickness.
Although prior art isolators have been made very compact, they are still much larger than other microwave elements. In particular, they are too high, protruding above surrounding elements. Thus, it has been desired to provide isolators with a much thinner construction.